$BC{l}_3$ does not exist as dimer but $B{H}_3$ exist as dimer $\left(B_2{H}_6\right)$ because

Chlorine is more electronegative than Hydrogen

There is back bonding in $BC{l}_3$ but $B{H}_3$ does not contain such multiple bonds

Large sized chlorine atom don't fit in between small boron atoms where as small sized hydrogen atom get fitted in between boron atoms.

Chlorine has high electron affinity.